The central hypothesis of this project is that long-term plasticity of the brain is mediated in large part by morphological reorganization of neuronal circuits, and specifically by the selective formation and elimination of synapses. Although this is a widely held belief, the turnover of synapses has been minimally characterized in vivo, and little is known about the developmental, environmental and activity-dependent factors that control synapse remodeling, especially in the intact animal. We will produce PSD-95-GFP transgenic mice and rats with fluorescently-labeled synapses for time-lapse imaging of synapse formation and elimination (turnover) in central neurons. We will measure the pattern and regulation of synapse turnover in vivo, with the goal of correlating these changes to plasticity and behavior. Neuron cultures from these transgenic animals will be used to investigate the molecular and physiological mechanisms that control synapse dynamics. Our specific aims are: 1) Create transgenic mice expressing PSD-95-GFP under a constitutive promoter to label excitatory synapses in brain; 2) Create transgenic mice expressing unstable PSD-95-GFP under an activity-inducible promoter to selectively label newly formed synapses in recently active neurons; 3) Create transgenic rats expressing PSD-95-GFP under constitutive and activity-inducible promoters, using newly developed lentiviral technology; 4) Characterize the dynamics of synapse turnover in visual cortex during postnatal development and its regulation by visual input, using the transgenic PSD-95-GFP animals; 5) Characterize the spatiotemporal patterns and underlying mechanisms of synapse turnover in vitro, using cultured neurons derived from the PSD-95-GFP animals. In collaboration with Tonegawa, Bear, and Liu, this proposal uses a multidisciplinary approach to explore synaptic interactions within neurons, and the morphological correlates of cortical plasticity and learning and memory. The project contributes to multiple objectives of the Center and depends on the combined scientific expertise and facilities of the Center.